With the development of digital telephony and with the rapidly increasing need for data distribution between centres and terminals over telecommunication lines, the question of the choice of 4-wire or 2-wire telecommunication has become of immediate interest. 4-wire operation has the technical advantage that communication in different directions takes place in different wire pairs, while 2-wire operation has the economical advantage that telecommunication systems already installed, such as private automatic branch exchanges (PABX) can be utilized. In the 2-wire case, where the transmitter and receiver communicate over the same wire pair there are differences, inter alia for the frame structure, from what is applicable to the 4-wire case.
The final standard for digital telecommunication will probably be 4-wire operation. However, 2-wire operation will be necessary during a long transition period, since practically all terminal cabling is 2-wire at the present time. A great problem during the transition period will be to supply the market with both types of terminal. It will also be expensive to gradually change over from 2-wire terminals to 4-wire terminals. One way of solving this problem is to provide for using the same terminal in both systems of operation.